Many animal epithelial tissues are colonized by benign, often essential, bacterial symbionts, such as those in the oral cavity and along the apical surfaces of the cells lining the intestinal and respiratory tracts. These tissues, which occur at the interface between the environment and the body, are also the sites of infection by many bacterial pathogens (e.g., Haemophilus influenzae and Neisseria meningitidis). In both pathogenic and cooperative associations the bacteria and host cells communicate most commonly through interactions of their surface molecules or through the secretion of other bioactive compounds. The model symbiosis between the epithelial tissues of the squid Euprymna scolopes and its Gram-negative, luminous bacterial associate Vibrio fischeri is being exploited as a system by which to characterize these cross-Domain cell-cell interactions and to provide insight into the critical differences between beneficial and pathogenic animal-bacterial associations. Recent studies of this system have shown that lipopolysaccharide (LPS), the principal surface molecule of Gram-negative bacteria and the product responsible for the most dramatic mammalian responses to infective bacteria, induces specific changes in the squid host's cell biology and morphology during the initial stages of the relationship. In addition, characterization of host responses to LPS has revealed that this molecule works synergistically with proteinaceous compounds exported by the symbiont. The types of host cell 'behaviors' induced by these V. fischeri-derived factors include: the production of mucus to facilitate colonization of susceptible tissues, and the onset of a morphogenetic program that transforms host tissues from a morphology characteristic of the uncolonized juvenile to one characteristic of the mature, adult association. Several other identified responses of host cells to V. fischeri may also be affected by these compounds. The goal of this interdisciplinary project is to characterize bioactive molecules of V. fischeri, specifically LPS and the peptides that work in concert with LPS, that mediate host tissue responses during the early phases of the squid-vibrio association. The specific aims of this proposal are to: (1) characterize the structures of wild-type V. fischeri LPS and the LPS derived directly from the symbiosis, as well as characterize LPS derived from specific LPS-synthesis mutants that are associated with compromised host responses; (2) determine the precise biochemical identity and activity of bacterial molecules that act synergistically with LPS; and, (3) determine what aspects of LPS structure play a role in conferring resistance to host antimicrobial peptides.